1. Field of the Invention
The present invention relates to an intake air control system for an internal combustion engine, and particularly to an intake air control system for an internal combustion engine having a valve operating characteristic varying mechanism for continuously varying a lift amount of at least one intake valve of the engine.
2. Description of the Related Art
Japanese Patent Laid-open No. 2005-171793 (JP '793) discloses a control system for an internal combustion engine having a valve operating characteristic varying mechanism for continuously varying a lift amount of intake valves and an electronically-controlled throttle valve. According to the conventional system disclosed in JP '793, in an engine operating condition other than an idling condition and a high-load operating condition, an intake air amount of the engine is controlled by varying the lift amount of the intake valve (a valve operating angle). If the idling condition is detected, an opening degree of the throttle valve is feedback controlled, according to a deviation between a target rotational speed and a detected rotational speed. Further, with respect to the lift amount of the intake valves, a feedforward correction is performed according to an on/off (application/removal) state of external loads on the engine (e.g., a load for actuating auxiliaries or a load changed by the range shift of an automatic transmission).
In the above-described conventional system, control of the intake air amount by the intake valve is switched to the throttle valve when the engine operating condition shifts from an operating condition other than the idling condition to the idling condition. Therefore, there is a possibility that a torque shock may be caused by switching control between the intake and throttle valves.
Further in the above-described conventional system, the lift amount of the intake valves is immediately corrected in the increasing direction when the external load is applied. However, there is a possibility in the conventional system that an excessive correction is performed to fluctuate the engine rotational speed since the intake air amount rapidly changes by varying the intake valve lift amount, as described in JP '793.